An estimated 40 to 50 million people in the United States suffer from allergies, and around half of American adults who use medications to control allergy symptoms take antihistamine pills. While these drugs can reduce sneezing, itchy eyes, congestion and other symptoms, they can cause side effects, particularly for those who take too much allergy medication.

Understanding Antihistamines

Allergies occur when the immune system identifies a harmless substance like pet dander or grass pollen as a threat. To protect the body, the immune system releases chemicals that trigger an inflammatory response.

Histamine is one of the key immune system chemicals involved in allergic reactions. Antihistamine medications work by interfering with the effects of histamine. While there are prescription versions of these drugs available, most people take over-the-counter (OTC) allergy medicines.

There are two main types of OTC allergy medications: first-generation and second-generation.

  • First-generation antihistamines include diphenhydramine (Benadryl), brompheniramine, dimenhydrinate (Dramamine) and doxylamine. These medications also act on a part of the brain responsible for controlling nausea, so they’re used to treat motion sickness in addition to allergies.
  • Second-generation antihistamines include loratadine (Claritin), cetirizine (Zyrtec) and fexofenadine (Allegra). These medications affect different areas of the brain and spinal cord to provide longer-lasting relief from allergy symptoms.

Among prescription antihistamine medications, hydroxyzine is a unique option. In addition to blocking the effects of histamine, this drug also interferes with activities of the brain chemical acetylcholine, which controls both arousal and involuntary muscle movements. Due to its actions, hydroxyzine has a calming effect on the body and mind and is sometimes prescribed for anxiety.

Dosing Recommendations: How Much Is Too Much Allergy Medication?

Both over-the-counter and prescription antihistamines have clear dosing instructions. For most first-generation OTC medications, dosing recommendations advise taking one or two pills every 4 to 6 hours. The instructions for second-generation OTC antihistamines typically state that a person should take one pill every 12 or 24 hours.

Whether you’re taking antihistamines for allergies, anxiety or motion sickness, following the dosage instructions is important to prevent an overdose. Read the packaging carefully and take only the number of pills suggested as often as recommended unless a health care provider advises otherwise.

Potential Side Effects: Can You Overdose on Allergy Medication?

Since antihistamines are available over the counter, you may think these medications won’t have side effects or pose any risk of overdose, but this isn’t the case.

Side Effects of First-Generation Antihistamines

Common side effects of first-generation antihistamines include:

  • Blurred vision
  • Constipation
  • Dizziness
  • Double vision
  • Drowsiness
  • Dry mouth
  • Headaches
  • Low blood pressure
  • Rapid heart rate
  • Thickening of mucus in the airways
  • Trouble urinating

Because first-generation antihistamines often cause drowsiness, people sometimes take them as sleep aids. Over time, the body can build up a tolerance to the medication, making it necessary to take more to feel drowsy. Taking more antihistamine than recommended to induce sleep can lead to overdose.

Some people also intentionally take too much allergy medication to experience a high. At high doses, diphenhydramine can cause delirium, seizures, psychosis and even death.

Side Effects of Second-Generation Antihistamines

Potential side effects of second-generation antihistamines include:

  • Abdominal pain
  • Cough
  • Fatigue
  • Headache
  • Nausea
  • Sore throat
  • Vomiting

People are less likely to abuse second-generation antihistamines because they don’t cause feelings of drowsiness the way first-generation allergy medications can. Most often, overdoses are accidental and occur when someone forgets they already took their daily dose. A single extra dose increases the risk of side effects but is unlikely to prove fatal.

As a result, Claritin, Allegra and Zyrtec overdose deaths are rare. For a fatal overdose of Zyrtec or another second-generation antihistamine to occur, a person would likely have to take a large number of pills.

Risk Factors for Antihistamine Overdose

Some people are at increased risk for antihistamine overdose due to other medications they take or medical conditions they have.

Other drugs that cause drowsiness can intensify the sedative effect of first-generation antihistamines. As a result, experts generally recommend that you don’t take these drugs with other sleep aids, muscle relaxants, sedatives, seizure medications and some antidepressants.

Diphenhydramine and narcotic pain relievers are an especially dangerous combination. From 2019 to 2020, antihistamines played a role in nearly 15% of overdose deaths, and the majority of these fatalities involved a combination of antihistamine and opioids. Diphenhydramine has also been shown to make the overdose rescue drug naloxone less effective, increasing the risk of death when a person overdoses.

Hydroxyzine can interact with all the drugs previously mentioned as well as other medications that suppress the activities of acetylcholine. As a result, the medication shouldn’t be taken with tricyclic antidepressants, certain antipsychotics and some anti-nausea drugs.

Certain medical conditions can also heighten the risk of side effects of antihistamines and the risk of overdose. Generally, people with diabetes and liver disease should avoid first-generation antihistamines, and individuals with kidney disease shouldn’t take second-generation medications.

Regardless of your health history, it’s wise to consult your medical provider before taking any OTC antihistamine for the first time. They can provide you with guidance on the proper dosage and advise you about potential side effects.

Seeking Medical Help

If you take too much allergy medication or suspect a loved one has done so, it’s important to seek medical help right away. Prompt medical treatment can greatly reduce the risk of death due to an overdose of Zyrtec, Benadryl or another antihistamine.

If the person is awake and responsive, you can typically contact Poison Control for advice on the next steps to take. When someone is unconscious or delirious due to an overdose, call an ambulance.

Get Support for Allergy Medication Misuse

If you’re concerned about the effects of taking too much allergy medication to control symptoms of anxiety or misusing antihistamines, help is available. Sunlight Recovery offers both substance use and mental health treatment programs, and our counselors are available 24/7 to explain your treatment options. Contact us today to learn more about how we can assist you.